Electrically operated audible alarm



Oct. 4, 1966 B. M. POTTER 3,277,465

ELECTRICALLY OPERATED AUDIBLE ALARM Filed Feb. 25, 1963 AMPLIFIERAMPLIFIE PHASE TRANSDUCER PHASE TRANSDUCER INvE E L :I INVERTER StatesThis invention relates to electrically operated audible alarms andsignals.

Most transducers for changing electrical energy into sound,loudspeakers, for example, have a frequency at which they no longerbehave as linear elements, but as a resonant element. This effect isoften described as tinniness or buzz, and every effort is made to reduceit. A device "with an extremely sharp critical frequency is the smallcrystal earphone used in inexpensive hearing aids and transistor radios.In such a device, a piezoelectric element is mechanically coupled to athin foil diaphragm. The masses involved, the mounting of thepiezoelectric element, and the tension of the diaphragm combine to forma system that is sharply resonant. Such a device can be designed to haveits resonant or critical frequency in the area of the ears maximumsensitivity.

The problem of producing audible signals suitable for alarms andindications from small electric currents has heretofore not been dealtwith effectively. In general, the problem is that the transducingelement requires a low impedance and high driving power in the vacuumtubes or transistors associated with it. The design of circuits is suchas to force the transducing element to adhere to the wave shape fed toit, and there is a loss of power in heat because of the energy devotedto overcoming the transducers characteristics.

In an alarm in accordance with the invention, a transducer, comprising adiaphragm mechanically coupled and driven by piezoelectric means, isincorporated in circuitry in such a way as to permit the natural rate ofvibration of the unit as a Whole to reflect back into the semiconductoroscillator circuit associated with it, even as the resonant length of anorgan pipe reflects back to the stream of air generating the note, thusto cause the alarm to oscillate in a mode of such wave shape andfrequency as to contain substantial energy at the critical frequency ofthe transducer.

In addition, in an alarm in accordance with the invention, theadvantages of -a peculiar effect can be realized, since good waveshapeis of no particular interest; just the opposite, in general, the moreragged the wave shape the more arresting the tone, the transducer can bepulsed with short-duration spikes of current and be made to operate atextremely high efiiciency.

In the accompanying drawings, there are shown illustrative embodimentsof the invention from which these and other of its objectives, novelfeatures, and advantages will be apparent.

In the drawings:

FIGURE 1 is a block diagram of one type of claim in accordance with theinvention,

FIGURE 1A is a block diagram of another embodiment of the invention,

FIGURE 2 is a view of circuitry that may be utilized in the type ofalarm shown in FIGURE 1,

FIGURE 3 is a like view of circuitry of the same type but including atransformer,

FIGURE 4 is a view of circuitry that may be utilized in the type ofalarm shown in FIGURE 1A,

FIGURE 5 is a view of a circuit for an alarm of the type shown in FIGURE1 permitting pulsing, and

FIGURE 6 is another circuit for that type of alarm at a certainfrequency, opposes the fiow of current less 3,277,465 Patented Oct. 4,1956 ice than at any other frequency so that oscillation is maintainedat this critical frequency. Biasing and coupling considerations arestandard in the transistor circuitry except a diode 14 at thephase-inverter-amplifier 13 permits somewhat louder operation at expenseof efficiency by providing a return path for positive voltages appearingat the return lead 15 of the transducer.

In FIGURE 3, the circuit 20 again shows the principles outlined inFIGURE 1. The phase inversion is accomplished by the transformer 21. Thetransducer 22 has been placed after the phase inverter 21 in thefeedback loop to take advantage of the transformers capacity to multiplythe voltages at the collector 23 of the transistor 24, for the purposeof loudness. The diode 25 completes the circuit for positive voltagesappearing at the return lead 26 of transducer 22, permitting louderoperation at the expense of efficiency.

In FIGURE 4, the generally indicated circuit 30 shows the principlesoutlined in FIGURE 1A. In the circuit 30, the transducer 31 is outsidethe feedback loop. In the loop, the transistor amplifier 32, and thetransformer 33 matches the output of the amplifier 32 to the input.Because the amplifier is of the common-collector type, no phaseinversion is necessary. Capacitor 34 provides coupling between thetransformer 33 and transducer 31.

Except for its biasing arrangements, the circuit 40 of FIGURE 5 whichincludes the transducer 41, is substantially identical to that shown inFIGURE 3. However, in the base circuit, a capacitor 42 has been placedin series with the diode 43 so that the positive voltages appearing atthe return lead 44 charge the capacitor 42 over a period of many cyclesuntil sufficient current passes through resistor 45 to halt oscillation.The charge in the capacitor leaks off through resistor 46 over a periodcorresponding to many cycles until oscillations can recommence.

The transducers 12, 22, 31 and 41 may all be the same. In any event,each is shown as including a diaphragm D and a piezoelectric element Emechanically coupled thereto as at F.

The circuit 50 shown in FIGURE 6 has a transducer 51 which has a drivingpiezoelectric element 52 mechanically coupled as at 53 to a sensingpiezoelectric element 54 which, in turn, is mechanically coupled as at55 to the diaphragm 56. In the circuit, the feedback and matching areaccomplished by the transducer. The driving element 52 is in the outputof amplifier 57 and the sensing element 54 is the input thereof. Thefeedback loop thus contains the coupling between the elements.

I claim:

1. An alerting device comprising a piezoelectric transducer having avibratory surface from which acoustic energy is radiated and apiezoelectric means operative to drive said surface, said transducerhaving a predetermined frequency of resonance and operable, whenvibrating substantially at said frequency, to radiate substantialaudible sound energy substantially within the frequency range of theears maximum sensitivity, a circuit for energizing said piezoelectricmeans, said circuit including amplifier means wth a transistor actuator,means for conducting energy from said amplifier to said piezoelectricmeans, a feedback path from said piezoelectric means to said amplifier,the circuit including means to provide phasing, biasing, and impedancematching to provide an oscillator whereby the energy input to saidpiezoelectric means is caused to oscillate substantially at saidresonant frequency of said transducer to provide an audible warning.

2. The alerting device of claim 1 wherein said piezoelectric meanscomprises a piezocrystal included directly in the oscillator feedbackloop.

3. The alerting device of claim 1 wherein said circuit includes meansfor modulating the oscillation of said circuit and the output of saidtransducer.

if References Cited by the Examiner UNITED STATES PATENTS OTHERREFERENCES The Radio Amateurs Handbook, 27th ed., pp. 47-51 (1950).

NEIL C. READ, Primary Examiner.

5 W. C. GL'EICHMAN, I. I. LEVIN, Assistant Examiners.

1. AN ALERTING DEVICE COMPRISING A PIEZOELECTRIC TRANSDUCER HAVING AVIBRATORY SURFACE FROM WHICH ACOUSTIC ENERGY IS RADIATED A PIEZOELECTRICMEANS OPERATIVE TO DRIVE SAID SURFACE, SAID TRANSDUCER HAVING APREDETERMINED FREQUENCY OF RESONANCE AND OPERABLE, WHEN VIBRATINGSUBSTANTIALLY AT SAID FREQUENCY, TO RADIATE SUBSTANTIAL AUDIBLE SOUNDENERGY SUBSTANTIALLY WITHIN THE FREQUENCY RANGE OF THE EAR''S MAXIMUMSENSITIVITY, A CIRCUIT FOR ENERGIZING SAID PIEZOELECTRIC MEANS, SIADCIRCUIT INCLUDING AMPLITUDE MEANS WITH A TRANSISTOR ACTUATOR, MEANS FORCONDUCTING ENERGY FROM SAID AMPLIFIER TO SAID PIEZOELECTRIC MEANS, AFEEDBACK PATH FROM SAID PIEZOELECTRIC MEANS TO SAID AMPLIFIER, THECIRCUIT INCLUDING MEANS TO PROVIDE PHASING, BIASING, AND IMPEDANCEMATCHING TO PROVIDE AN OSCILLATOR WHEREBY THE ENERGY INPUT TO SAIDPIEZOELECTRIC MEANS IS CAUSED TO OSCILLATE SUBSTANITALLY AT SAIDRESONANT FREQUENCY OF SAID TRANSDUCER TO PROVIDE AN AUDIBLE WARNING.